One interesting sound change in Old Latin was the change of /dw/ into /b/.

For example, bellum & #39;war& #39; (from which English gets & #39;bellicose& #39; and & #39;belligerent& #39;) was once duellom.

However, duellom did survive as an archaism, most often in poetry, and is the origin of English & #39;duel& #39;.

The modern meaning of & #39;duel& #39; as a fight between two parties (rather than an all-out war) is probably due to a medieval association of the word with the number two, which in Latin is duo.

Other words were affected by the change of /dw/ to /b/.

These include the common adjective bonus & #39;good& #39; - in Old Latin, the word was duenos.

Duenos in fact ppears in one of the oldest Latin texts, the Duenos inscription.

Why does English have two-related words that begin with du- (dual, duo, duplicate) and with bi- (binoculars, bisexual, bifocal)?

Because of this change!

The first group get du- straight from Latin duo & #39;two& #39;, while bi- comes from bis & #39;twice& #39;, which was originally duis.